It is well known that the term "color" as applied to electromagnetic radiation represents in part the relative energy distribution of the radiation within the visible spectrum. That is, light providing a stimulus to the human eye, and having a particular energy distribution, may be perceived as a substantially different color than light of another energy distribution. Concepts relating to the characteristics of color and light waves are the subjects of numerous well known texts, such as Principles of Color Technology, Billmeyer, Jr. and Saltzman (Wiley 1966).
In recent years, the capability of maintaining the "quality" of color has become of significant importance in various industries, such as, for example, the fields of graphic arts, photography and color film processing. With respect to the graphic arts fields, it is necessary, for example, to maintain appropriate color quality throughout a production run of a color printing sheet.
For purposes of performing sample testing and other activities in furtherance of maintaining color quality, it is necessary to first determine an appropriate means for "measuring" and "describing" color. A substantial amount of research has been performed during the past 50 years with respect to appropriate methods and standards for color measurement and description.
For purposes of describing color, and from a purely physical point of view, the production of color requires three things: a source of light, an object to be illuminated, and a means for perceiving the color of the object. The means for perceiving the color can be the human eye and brain or, alternatively, a photosensitive detector and associated auxiliary equipment utilized for detecting light.
The maintenance of quality standards in photography requires precise control of exposure, source intensity, development procedures and film characteristics, in addition to the control of environmental variables. Similarly, the maintenance of quality standards in graphic arts also involves consideration of some of the same parameters and variables. In general, it is desirable to provide a means for measuring color so as to assess the manner in which an image will appear to a human observer, or the manner in which an image will perform in a photographic or other type of reproduction printing operation.
One parameter widely used in the field of color technology for obtaining a quantitative measurement is typically characterized as optical "density." Described simplisticly, when light is directed onto an object or object sample to be measured for color, the object may absorb a portion of the light energy while correspondingly passing through or reflecting (if the object is opaque) other portions of the light. The color characteristics of the object sample will depend in part on the spectral characteristics of the object. That is, the effect of an object on light can be described by its spectral transmittance or reflectance curves (for transparent or opaque materials, respectively). These spectral characteristic curves indicate the fraction of the source light at each wavelength transmitted by or reflected from the materials. Such curves are a means for describing the effect of an object on light in a manner similar to the use of a spectral energy distribution curve for describing the characteristics of a source of light.
For purposes of determining these spectral characteristics, a detector can be appropriately positioned to respond to the light transmitted through or reflected by the object sample. Such a detector can, for example, be in the form of a photovoltaic device. Such a device can produce a current output proportional to input light intensity over several orders of magnitude.
In accordance with conventional optical physics, it is known that the proportion of light incident to an object sample and absorbed by such a sample is independent of the light intensity. Accordingly, a quantitative indication of the spectral characteristics of an object sample can be defined as the transmittance or reflectance of the sample. That is, the transmittance of a substantially transparent object can be defined as the ratio of power transmitted over light power incident to the sample. Correspondingly, for an opaque object sample, the reflectance can be defined as the ratio of power reflected from the object over the incident light power. For collimated light, these ratios can be expressed in terms of intensities rather than power. Furthermore, because of the nature of transmittance/reflectance and the optical characteristics of the human eye, it is advantageous to express these ratios in logarithmic form. Accordingly, the optical density of an object sample is typically defined as the negative logarithm to base 10 of the transmittance or reflectance. In accordance with the foregoing, if an object sample absorbed 90% of the light incident upon it, and the object were opaque, the reflectance would ideally be 10%. The density of such a sample would then be characterized as unity. Correspondingly, if 99.9% of the light were absorbed, the reflectance would be 0.1% and the density would be 3. Similarly, the density of an "ideal" object reflecting 100% of the light incident upon it would be zero.
To provide a relative measurement of color, it is possible to utilize the principles of density determinations without requiring measurement or knowledge of the absolute values of total incident light intensity or reflectance. That is, for example, it is possible to obtain relative color measurements among a series of object samples by utilizing a particular geometric configuration of light, object sample and reflectance or transmittance detector for each measurement, and standardizing the measurements in some desired manner.
In brief summary, optical density is a measure of the modulation of light or other radiant flux by an object sample, such as a given area of printed ink-on-paper. Density measurements provide a means to assess the manner in which an image will appear to a human observer, or the way an image will perform in a printing operation. Density measurements can be utilized to produce sensitometric curves to evaluate various printing and reproduction characteristics, as well as utilization to control various photographic operations, such as film processing.
For purposes of measuring optical densities, it is well known to employ a device typically characterized as a densitometer. For purposes of further description of the background of the invention, additional discussion will be limited to principles associated with "reflection" densitometers, which are employed for optical density measurements of opaque objects. However, it should be emphasized that the principles of the invention are not limited to reflection densitometers, and can readily be applied to other types of devices (such as transmittance densitometers) employed for determining the spectral characteristics of various non-opaque materials.
Reflection densitometers are utilized in the graphic arts for performing a variety of functions. As an example, it is common to provide color printing sheets with color bar strips extending along an edge of the sheet. When such a printing sheet has been approved for production, the optical color density of the color bars can be determined with the densitometer. Thereafter, during production runs, the color bars on the edges of corresponding printed sheets can be checked with the densitometer, so as to assure that appropriate color densities are being maintained.
In addition, reflection densitometers can be employed in the area of photography. For example, such a densitometer can be utilized to determine the optical density of the brightest or "highlight" areas, and the darkest or "shadow" areas of of a subject to be photographed. Such values can be utilized in adjusting controls of the camera so as to assure appropriate exposure.
Still further, reflection densitometers can be conveniently employed in color film processing. It is common for color film manufacturers to provide test strips having color bars. If the test strips have been appropriately processed, the bars will have known densitometer readings. Such strips can then be utilized to check operating parameters of a film processing system, before the system is utilized to process the exposed film.
To assist in describing the principles of the invention, presently known techniques of measuring optical density can be illustrated by the schematic representation of a known reflection densitometer configuration 100 as shown in FIG. 1. Referring to the numerical references therein, the prior art reflection densitometer 100 includes a light source unit 102 having a source light 104. With respect to optical density measurements in photography and other industrial fields, various standards have been developed for densitometer illuminating light sources. For example, densitometer standards have previously been described in terms of a tungsten lamp providing an influx from a lamp operating at a Planckian distribution of 3000K. Other suggested standards have been developed by the American National Standards Institute ("ANSI") and the International Organization for Standardization ("ISO"). These light source densitometry standards are typically defined in terms of the spectral energy distribution of the illuminant.
The source light 104 is directed through a collimating lens 106 which acts to converge the electromagnetic radiation from the source light 104 into substantially parallel rays of light. The light rays transmitted through the lens 106 are further directed through an aperture 108. The dimensions of the aperture 108 will determine the size of the irradiated area of the object sample under test. Various standards have been defined for preferable sizes of the irradiated area. Ideally, the aperture 108 would be of a size such that the irradiance is uniform over the entire irradiated area. Current standards suggest that the size of the irradiated area should be such that irradiance measured at any point within the area is at least 90 percent of the maximum value.
The light rays transmitted through aperture 108 (illustrated as rays 110 in FIG. 1) are projected onto the irradiated area surface of the object sample 112 under test. The sample 112 may be any of numerous types of colored opaque materials. For example, in the printing industry, the sample 112 may be an ink-on-paper sample comprising a portion of a color bar at the edge of a color printing sheet. However, as will be apparent from the subsequent description herein, the principles of the invention are not limited to measurement of printed ink-on-paper.
As the light rays 110 are projected onto the object sample 112, electromagnetic radiation shown as light rays 114 will be reflected from the sample 112. For purposes of determining the relative proportions of light reflected from various object samples, it is necessary to obtain a quantitative measurement of this reflected light. However, it is undesirable (and substantially impossible) to measure all of the light reflected from the sample 112. Accordingly, standard detection configurations have been developed whereby reflected light is detected at a specific angle relative to the illumination light rays 110 projected normal to the plane of the object sample 112. More specifically, standards have been developed for detection of reflected light rays at an angle of 45.degree. to the normal direction of the light rays 110.
For purposes of actual detection of the reflected light rays 114, a rotatable spectral filter apparatus 116 is provided. The filter apparatus 116 can include a series of filters 118, 120 and 122 which are employed for purposes of discriminating red, green and blue spectral responses, respectively. That is, each of the filters will tend to absorb light energy at frequencies outside of the bandwidth representative of the particular color hue of the filter. For example, the red filter 118 will tend to absorb all light rays except for those within the spectral bandwidth corresponding to a red hue and centered about a wavelength of approximately 610 nanometers (nms). By detecting reflected light rays only within a particular color hue bandwidth, and obtaining an optical density measurement with respect to the same, a "figure of merit" can be obtained with respect to the quality of the object sample coloring associated with that particular color hue.
It is apparent from the foregoing that the actual quantitative measurement of color density or reflectance is dependent in substantial part on the spectral transmittance characteristics of the filters. Accordingly, various well known standards have been developed with respect to spectral characteristics of densitometer filters. For example, one standard for densitometer filters is known as the ANSI Status T Color Response. The spectral response characteristics of filters meeting this standard are relatively wideband (in the range of 50 to 60 nanometer bandwidth) for each of the red, blue and green color hues. Other spectral response characteristic standards include, for example, what is known as G-Response, which is somewhat similar to Status T, but is somewhat more sensitive with respect to denser yellow hues. An E-Response represents a European response standard.
The spectral filter apparatus 116 shown in FIG. 1 includes not only the filters 118, 120 and 122, but is also shown as including a shaft 124 having one end connected to a "wheel" 126 on which the spectral filters are positioned and spaced apart. The other end of the shaft 124 is connected to a manually rotatable knob 128. In the actual mechanical configuration of the densitometer 100, the knob 128 would be made accessible to the user for purposes of manual rotation of the wheel 126 so as to selectively position the individual filters as desired. In FIG. 1, the red filter 118 is shown as being appropriately positioned for detecting the reflected light rays 114.
The spectral filters 118, 120 and 122 can be any of several specific types of spectral response filters. For example, the filters 118, 120 and 122 can comprise a series of conventional Wratten gelatin filters and infrared glass. However, various other types of filter arrangements can also be employed.
As further shown in FIG. 1, the portion of the reflected light rays 114 which pass through the filters of the spectral filter apparatus 116 (shown as light rays 130) impinge on a receptor surface of a photovoltaic sensor cell 132. The sensor 132 is a conventional photoelectric element adapted to detect the light rays 130 eminating through the particular one of the filters 118, 120 and 122 then positioned to receive the reflected light rays 114. The sensor 132 is further adapted to generate an electrical current on line pair 134, with the magnitude of the output line current being proportional to the intensity of the light rays 130 sensed by the sensor 132. Photoelectric elements suitable for use as sensor 132 are well known in the art and various types of commercially available sensors can be employed.
The sensor current output on line pair 134 is applied as an input signal to a conventional amplifier 136. The amplifier 136 serves to convert the electrical current signal on line pair 134 to an output voltage signal on line 138. The amplifier 136 can include gain adjustment circuitry (representatively shown as an adjustable resistance in FIG. 1) 139 for purposes of varying the output voltage to input current gain. For example, a standard may be defined for the densitometer density reading for a particular spectral filter for zero density level. Accordingly, the amplifier circuit 136 can be adjusted by means of the gain adjustment circuitry 139 so that the densitometer reading is appropriate for the standard.
The output voltage signal from the amplifier 136 on line 138 can be applied as an input signal to a logarithmic voltage converter 140. The logarithmic voltage converter 140 is adapted to provide an output on line 142 which corresponds to the optical density measurement for the object sample 112 and the particular configuration of the spectral filter arrangement 116. This optical density measurement may be in the form of the negative logarithm (to the base 10) of the ratio of the voltage signal on line 138 to a standardized voltage magnitude. This standardized voltage magnitude can be set to a value which the user wishes to have correspond to a zero optical density measurement. That is, if the output voltage on line 138 is equal in magnitude to this standardized value, the logarithmic computation provided by the logarithmic converter 140 would generate a density measurement on line 142 of zero.
Preferably, the logarithmic converter 140 also has gain adjustment circuitry 144. This gain adjustment circuitry 144 can be utilized to set the density "slope" sensitivity of the converter 140. As is well known in the art of densitometer circuit design, logarithmic converters can vary in their response characteristics to input voltages. The gain adjustment provides a means for adjusting the response characteristics.
The voltage output from the logarithmic voltage converter 140 on line 142 can be applied to any of numerous types of conventional display apparatus 146. The display apparatus 146 is utilized to provide a visual display to the user of the density measurement represented by the logarithmic converter output voltage on line 142.
Although the foregoing prior art densitometer 100 has been described with the logarithmic conversion and gain adjustment functions represented by discrete components, it will be apparent that such functions can clearly be performed by means of a digital computer or other computer apparatus.
As is well known in the art, densitometer apparatus must first be "calibrated" to provide a desired density response characteristic for a given set of spectral filters. In known systems, for example, and as briefly discussed in previous paragraphs, the "zero density" condition and the response "slope" for a particular densitometer and filter set can be provided as parameters manually inputted to the densitometer. For example, to provide what can be characterized as an "initial condition" of zero density for each individual spectral filter, an object sample comprising a "white" reference patch (representing substantial reflection) can then be measured for each of the individual filters. The densitometer gain adjustments can then be manually adjusted so as to provide a standardized densitometer reading for the patch. Correspondingly, with the logarithmic density measurement assumed to be linear, the "slope" of the densitometer response can be set by means of viewing a "black" patch (representing substantial absorption), and setting the densitometer reading to a standardized "maximum" for the patch measurement for each of the filters.
Although the foregoing represents a means for calibrating zero density level measurements and density slope sensitivity, the known systems employing these calibration procedures still suffer from several substantial disadvantages. First, when standards are provided for adjusting the density level readings for particular filter types, the standards assume an "ideal" filter. However, any physically realizable spectral filter arrangement will vary from the ideal. For example, in a conventional Wratten filter configuration, such errors may be within the range of .+-.5 nanometers. Such filter manufacturing errors can correspondingly result in errors as large as .+-.0.08 Density units in measurement of certain printed ink types. Such errors are critical, since desired industry inter-instrument agreement is within .+-.0.02 Density.
In addition, historical data regarding density measurements can be of primary importance, especially within the printing industry. That is, all printing being performed within a singular controlled environment should be capable of measurement by a number of densitometers in a manner so that the same results are achieved for identical measurements. However, if a series of conventional densitometers were utilized to measure the same color area, and were calibrated in accordance with the previously described procedures, the densitometers would not display identical measurement readings. Accordingly, if one densitometer had been used for an extensive period of time and had generated important historical printing data, such data would be substantially useless if the densitometer malfunctioned and a second densitometer instrument was subsequently utilized.
Problems associated with previously known calibration procedures result from several other considerations, in addition to the problems associated with manufacturing tolerances of spectral filter arrangements. For example, specification standards for various types of spectral filter arrangements call for certain types of light and color temperature, in addition to other illuminant parameters. However, manufacturing errors exist with respect to all physically realized illuminants. Furthermore, as a densitometer is used over a period of time, filament lamps will tend to drift. Still further, manufacturing errors will tend to exist with respect to photovoltaic detectors and other densitometer components. All of these factors result in problems associated with calibration based on standard spectral responses and the use of multiple densitometers for measuring color within a singular environment.